A typical valve includes a valve body having a movable valve member internally disposed therein. The movable valve member is positioned in a fluid flow passage extending between an inlet and an outlet of the body for selectively controlling fluid flow through the flow passage. Movement of the internally disposed valve member is usually effectuated by an actuating shaft interconnecting the valve member with an actuating mechanism, either manual or automatic, positioned externally of the housing. The actuating shaft is generally rigidly affixed to the valve member for common movement therewith.
In order to permit both assembly of the movable valve member within the body and subsequent maintenance, the valve body generally has an access opening which is closable by a removable valve cover. The actuating shaft may, as is the usual case, extend through an aperture in this cover. Fastening elements, most generally threaded members such as bolts or screws, may be used to removably secure the cover to the valve body. Screws, for example, are commonly used in an arrangement in which they extend through the cover and are received by threaded openings in the valve body to releasably join the cover to the valve body. Alternatively, bolts extending through aligned apertures in the valve cover and flanges on the valve body are used in another common arrangement for securing valve covers to valve bodies. The bolts may be removably held by nuts on the opposite flange side.
When used in a pressurized system, a valve is internally subjected to the pressure of the fluid process media flowing through the valve. This process media pressure tends to separate the valve cover from the valve body and imparts a tensile stress on the fastening elements connecting these two elements. In high pressure applications, the process media pressure, and the resulting tensile stress on the fasteners, may be quite substantial. Under standard engineering practices, fasteners of sufficient size and strength are selected to withstand tensile stresses well in excess of those to which the fasteners are expected to be subjected in operation.
Unfortunately, fasteners occasionally have material faults which excape detection, even when high quality inspection standards are employed. Such faults may, for example, take the form of a microscopic cracks in the material, and will substantially reduce the strength of the fastener material. Since tensile stresses tend to pull the material apart and accentuate microscopic cracks, such faults make fasteners under tensile stress especially prone to failure.
In addition to the problems of improper design and inadequate quality standards, many materials, including metals commonly used for bolts, screws and other similar fasteners, are subject to a phenomenon known as stress corrosion cracking. Although the mechanics of this phenomenon are not fully understood, stress corrosion cracking appears to result from exposure of the materials to particular elements. Chlorine and chlorine compounds, for example, attack certain stainless steel materials and cause the materials to fail under stress conditions. Other elements, such as sulfides, attack other metals and result in stress corrosion cracking. Even trace amounts of the attacking elements can cause the stress corrosion cracking phenomenon.
High performance materials, such as bolts or screws under high tensile stress, are particularly susceptible to stress corrosion cracking. The problem is particularly pronounced in high pressure valves located in chemical processing plants where many of the attacking elements, airborne or otherwise, come into contact with the cover fasteners and where the high pressure of process fluids commonly impart substantial tensile stress to bolts, screws or other types of fasteners securing a cover to a valve body. If the pressure of the process media exceeds the tensile strength of the fastener material, the fasteners will fail. Injury or death to persons working in proximity to the failed valve may result. To make the problem worse, stress corrosion cracking is not detectable from visual inspection and a fastener suffering from stress corrosion cracking may appear completely normal immediately prior to failure.